We recently demonstrated that different bacterial communities established in the intestine by breast vs. bottle feeding are associated with different trajectories of immunologic development. Furthermore, the immunologic characteristics found to distinguish breast- and bottle-fed individuals are those thought to be important for protection against HIV transmission and disease progression. Such differences may also explain the variable protective efficacy of candidate HIV/SIV vaccines. Scientists investigating another vaccine, trivalent influenza vaccine, demonstrated the importance to vaccine responses of TLR signaling from intestinal bacteria. We hypothesize that intestinal bacteria have two separate effects on vaccine responses of infants: (i) an effect on the magnitude of the elicited response, which is determined at the time of vaccination by a signal delivered by intestinal bacteria to the host, and (ii) an effect on the shape of the host immune system that has implications for the race between host and virus in the earliest stages of disease, i.e., for the ability of the elicited response to be protective. Our specific aims are: 1. To determine if differnt bacterial communities present in breast- and bottle-fed infants result in different levels of response to RhCMV-vectored vaccine. Breast- and bottle-fed infants will be vaccinated either (i) at six months of age, when they have different bacterial communities but similar immune systems, or (ii) at one year, when the bacterial populations have become more similar but immune systems have diverged. Our hypothesis suggests that intensity of the immune response to vaccine will be determined largely by bacteria in the gut. 2. To test the impact of different microbiotas on the earliest stages of viral replication. Gut commensal bacteria might impact upon vaccine protectiveness by effects on the magnitude or quality of vaccine-specific responses, on the developmental trajectory of immune subsets, or by changing viral replication at the earliest stages after infection. We will assess the impact of host microbes on early viral replication by instilling these microbes into gut loops. 3. To determine if developmental qualities of the host immune system conferred by gut bacterial communities facilitate vaccine-mediated protection against disease. At 14 months of age, individuals vaccinated in Aim 1 will be challenged by repeated low-dose exposure. At this time point the breast- and bottle-fed groups have been weaned, their bacterial populations are similar, but their immune systems remain clearly distinct. We predict that the level of protection achieved, for a given level of response t vaccine, will depend on developmental qualities of the host immune system conferred by the gut bacteria. In particular, the breast feeding-associated immune system will be associated with superior protection.